The contamination of soils with lead (Pb) has created major remedial, legal and policy problems in the U.S. and throughout the world. Models aimed at quantifying actual exposure, such as EPA's Lead Biokinetic Model are imprecise because they utilize assumptions concerning Pb bioavailability, i.e. the fraction of Pb absorbed from an ingested dose. This study aims to precisely determine the bioavailability of Pb in soils for representative sites in the U.S., which contain negligible amounts of other hazardous chemicals using isotopic techniques.